Printed circuit plates which are used much in various electronic instruments in these days are mostly produced by a so-called subtractive process, in which as metal leaf is laminated on the surface of an insulator base of a soft base, such as flexible polyester film or polyimide film, or of a hard base, such a paper-phenol base or a glass-epoxy base, then an etching resist coated film is formed on the surface of the metal leaf on the base by a screen printing method or a photo-processing method, and thereafter the unnecessary metal leaf in the non-resist area is dissolved and removed with an etching liquid to give a desired pattern on the base.
As the metal leaf to be on the surface of the base, mostly used at present is copper. However, aluminum has become used partly, because of the reasons that it has a lighter weight than copper, that it has excellent flexibility, that it has excellent corrosion resistance even under no corrosion-resistant treatment such as metal plating and that it is inexpensive.
Heretofore, as an etching liquid for aluminum, an aqueous solution of a metal chloride such as ferric chloride or cupric chloride, or an inorganic acid such as hydrochloric acid or phosphoric acid, or an aqueous solution of an inorganic alkali substance such as sodium hydroxide has been used. Of them, an aqueous solution of ferric chloride is used most popularly; and a concentrated phosphoric acid or an etching liquid essentially consisting of it is used for obtaining finer patterns.
However, where aluminum is etched with the above-mentioned conventional etching liquid, there occur various problems that too much under-cutting causes extremely thin lines of etched patterns and that too extremely rough edges of etched pattern lines cause much cutting of the lines. In addition, where a rolled aluminum leaf is used, since the leaf involves orientation of the crystal lattice configuration of the aluminum metal due to stretching during manufacture of the leaf, there occur still other problems that the etched condition differs in the pattern in the direction parallel to the stretched direction and in the pattern in the direction perpendicular to it and that damage of the resist coat film or adhesion failure of the film during etching gives defective patterns with lost areas. Where etching is effected with the conventional etching liquid, the edges and walls of the etched patterns would have large and small projections and depressions to be porous, in which the adhered etching liquid or halogens or other harmful ions to be contained in the materials to be used in the etching and other steps would remain therein as they are, without being completely removed in the successive washing step, and the remained components would accelerate corrosion of aluminum and would cause other various troubles. In particular, where printed circuit plates as produced by such conventional etching are combined into electronic instruments as LSI or the like, they would cause various troubles of the electronic instruments.
The above-mentioned drawbacks of conventional etching are not only in etching of aluminum but also in etching of copper, but the frequency of them is much more in the former than in the latter. The reasons are considered to be because of the obstructions specific to aluminum, such as the presence of an oxide layer of aluminum, the formation of local cells during etching and additionally the generation of hydrogen during etching.
Regarding the presence of the oxide layer of aluminum mentioned above, aluminum is naturally oxidized extremely with ease so that the surface of an aluminum leaf is to have an aluminum oxide layer thereon. The aluminum oxide layer on the surface of the leaf has a much lower dissolution rate by etching than the metal aluminum in the inside thereof. In producing printed circuit plates, the oxide layer on the surface of the aluminum leaf is generally removed by physical or chemical treatment prior to etching, but removal of the back side oxide layer as adhered to the insulator base is impossible by the treatment. Accordingly, as shown in FIG. 1, much time is needed for dissolution of the back side oxide layer in the terminal stage of etching, whereupon dissolution of the metal aluminum of both sides progresses much. It is considered that this would cause great under-cutting by etching to result in formation of noticeably thinned lines.
Next, regarding the formation of local cells during etching, aluminum having a lower standard electrode potential (or having a higher ionization tendency) than hydrogen generates much hydrogen during etching of it when it is dissolved in an etching liquid of an acid, alkali or chloride. This is because a metal aluminum having a high ionization tendency is ionized and dissolved whereby H.sup.+ is reduced to be H.sub.2. Thus, the etching mechanism of aluminum is different from that of copper having a low ionization tendency in this point. In the case of etching aluminum under such mechanism, where aluminum is brought into contact with a metal having a lower ionization tendency than it, it forms local cells to accelerate the ionization of itself much more, or that is, the aluminum becomes more soluble in the etching liquid. Accordingly, for example, where an aqueous solution of ferric chloride is used as an etching liquid in the case, the aluminum having a higher ionization tendency is ionized and dissolved, while the ferric ion is deionized to give an iron precipitate. Where the precipitated iron adheres to the surface of the aluminum, as shown in FIG. 2, it forms a local cell to further promote the dissolution rate of the adjacent aluminum. As a result, the aluminum is not dissolved at a uniform rate as a whole but is dissolved unevenly to give rough projections and depressions on the edges of the lines of the etched patterns or on the walls of them, as shown in FIG. 3, so that the etched pattern lines are to have extremely thinned areas and would be cut in some places.
An ordinary aluminum leaf contains, as impurities, metals having a low ionization potency, such as iron, silicon or copper. Therefore, formation of such local cells would occur not only in etching aluminum with an etching liquid containing a metal of a different kind such as ferric chloride or cupric chloride but also in the same with an etching liquid not containing a metal of a different kind such as hydrochloric acid or phosphoric acid. In addition, since such metals of impurities are not uniformly in an aluminum leaf but are locally around the crystal lattice of the aluminum metal, only the aluminum adjacent to the localized impurities is dissolved more rapidly so that dissolution of the aluminum is to be uneven. Further, the metals of impurities which have been dissolved would repeat the precipitation and dissolution, like the above-mentioned ferric chloride, to be seeds of forming local cells. It is said that formation of the local cells to cause the etching failure would occur not only by the use of metals of different kinds in etching but also due to the disorder of the crystal lattice configuration of the aluminum metal to be etched.
Regarding the obstruction due to generation of hydrogen, when aluminum is etched, a large amount of hydrogen is generated irrespective of the acid or alkaline etching liquid to be used, and the hydrogen gas has a peeling power to damage the edges of the resist coat film or bubbles of the gas temporarily adhere to the surface of the aluminum to prevent the surface from being in contact with the etching liquid. Thus, the generated hydrogen causes etching failure.